What is that substitute? It has to have the same energy density as gasoline and diesel, an energy return that is as high as that of pre-1970s oil (equivalent to 100 barrels extracted for every barrel used), and provide petrochemicals.
Ecological footprint involves multiple measurements.
Your third claim is illogical because of population momentum. Also, there is nothing in what you’ve shared so far that shows that 11 billion is sustainable, or even lower. In fact, the same footprint shows that even the current population is barely sustainable.
There is no need to campaign for not having children because, as this thread shows, that’s already happening due to prosperity. The problem is that population momentum is still taking place even with it, and increasing prosperity negates whatever resource gains are achieved through population aging.
Finally, for similar reasons, it is pointless for activists to call for population control because poverty is what leads to larger families and prosperity is what leads to the opposite. That’s why birth rates are higher in poor countries than in richer ones.
The problem is that resource consumption is also higher due to prosperity. In fact, this was shown in the references to the Netherlands and Nigeria, where ecological footprint for the former is five times higher. If everyone tried to copy the former, we’d need at least one more earth.
About your last point, I agree, but you’re failing to see the interplay between population, poverty reduction, and resource availability. Population increase goes down when there’s poverty reduction, but that also leads to population aging. At the same time, poverty reduction means using more resources and energy per capita, which means we see the same effects as we would with increasing population. That’s why we have countries like the Netherlands, which face population aging but also use resources per capita many times more than most countries, and Nigeria, which has increasing population but also low resource use per capita per poverty.
Are you now beginning to see the complexity of this issue?
If you want to see that further, see it in light of your reference to Malthus. He lived during a time when industrial civilization was just taking off, which is why world population grew at a faster rate, until it reached 2 billion by 1945. But after that, the population shot up to 6 billion in only a few decades, many times faster than it did for thousands of years. Would you like to guess why?